This invention relates to an apparatus and method of focusing two different wavelengths, such as wavelengths in the visible region (i.e., a camera) and in the ultraviolet region (i.e., a laser), to approximately the same focal plane. More particularly, this invention relates to an apparatus and method to test electrical traces wherein a visible wavelength and an ultraviolet wavelength are focused to a common focal plan.
In applications where two or more wavelengths, such as ultraviolet light (200-400 nm) and visible light (500-600 nm), are focused to the same focal plane (i.e., correction of a chromatic aberration), a lens system of different transparent materials is used in lens construction. Conventionally, for ultraviolet and visible light systems, the materials of choice are calcium fluoride and silica. Because calcium fluoride is costly and difficult to obtain and does not provide adequate image quality, an alternative to the conventional lens system would be beneficial.
U.S. Pat. No. 4,770,477 to David R. Shafer, U.S. Pat. No. 5,260,578 to Bliton, et al., and U.S. Pat. No. 5,969,883 to Yamakawa, et al. disclose multiple element lens systems to correct for various imperfections, including chromatic aberrations. Each of these references disclose the use of at least one element made of costly calcium fluoride. These U.S. Patents are incorporated herein by reference in their entirety.
Commonly owned, co-pending U.S. patent application Ser. No. 09/231,410 (U.S. Ser. No. ""410) and U.S. patent application Ser. No. 09/461,801 (U.S. Ser. No. ""801), incorporated herein by reference in their entirety, describe a method and an apparatus, respectively, in which an achromatization alternative is necessary. These patent applications collectively disclose an apparatus and method in which electrical traces can be detected in a circuit board without physical contact or electrical connection. In the current manufacture of electronic systems, the packing density of system components has increased considerably, requiring extremely small circuit traces on the order of 0.002 to approximately 0.003 inches. Fabrication of these traces is difficult and defects such as opens or shorts are common. Therefore, testing the integrity of fine traces has become increasingly more important. Most conventional methods of trace testing involves physically contacting the trace with one or two test probes. The physical placement accuracy of test probes limit their use in testing in large quantities. Most significantly, the speed of testing is limited by the density of the traces and corresponding probes as well as the setting time. Many current traces are so small or densely packed that they can only be contacted individually with a physical probe in a very time consuming and uneconomical visual process.
U.S. Ser. No. ""410 and U.S. Ser. No. ""801 fulfill the need for a test method for fine traces in which the trace is not physically contacted and which is not unduly time consuming. The apparatus disclosed in U.S. Ser. No. ""801 employs a camera (representing visible light) and an electromagnetic source (representing ultraviolet light) directed at the circuit board. For proper operation, the camera and electromagnetic wavelengths must be focused to substantially the same focal plane. Existing lens systems that fulfill this need often employ calcium fluoride elements and do not produce the image quality required for the proper and efficient operation of the testing system.
Accordingly, there exists a need for the development of a method and apparatus that achieves the same end result of the conventional calcium fluoride and fused silica lens system. More specifically, there exists a need for a method and apparatus that focuses two different wavelengths to a common focal plane with improved image quality.
Accordingly, an object of the present invention is to provide a method and apparatus for testing circuit boards wherein the scanning camera and the ultraviolet laser source are focused to a common focal plane.
The present invention is particularly useful in a tester for testing an electrical trace that provides for common focusing of an ultraviolet light source with a scanning camera using a fused silica objective scan lens. This system is generally described in commonly owned U.S. Ser. No. 09/231,410 and U.S. Ser. No. 09/461,801 and includes a scanning path camera sensitive to wavelengths in the visible region that is focused to a predetermined off-set. The scanning camera is directed toward the trace under test. An ultraviolet laser source is also directed at the test trace. A fused silica objective scan lens is placed in the path of the scanning camera and the ultraviolet laser source. For proper functioning, the two wavelengths (i.e., the ultraviolet wavelengths and the visible wavelength) must be substantially commonly focused on the board under test.
In a first embodiment, a method of testing an electrical trace is described. In this embodiment, an ultraviolet laser source is focused on a board having a test trace. A scanning camera sensitive to wavelengths in the visible region and focused to a predetermined off-set is directed at the test trace. An auxiliary lens is then placed in the path of the scanning camera prior to the introduction of the ultraviolet laser source. This auxiliary lens has a power sufficient to bring the camera into focus on the test trace. Accordingly, this auxiliary lens is chosen to have a power sufficient to accommodate the difference between the predetermined off-set and the test trace, taking into account the fused silica object scan lens which is placed in the path of the ultraviolet laser source and the scanning camera.
In a second embodiment of the present invention, the scanning camera and the ultraviolet laser source are commonly focused by shifting the camera optics until the focal plans agree. More specifically, an ultraviolet laser source is focused on a test trace. The scanning camera is focused to infinity and is directed at the test trace. A fused silica objective scan lens is placed in the path of the ultraviolet laser source and the scanning camera. The scanning camera has camera optics that may be moved independently of its visible light source. Accordingly, the scanning camera optics is moved relative to the camera""s visible light source until the camera commonly focuses with the ultraviolet laser source at the test trace. One skilled in the art would recognize that the degree (e.g., the offset) that the camera optics is shifted depends on the dimensions of the overall system, particularly the objective scan lens placed in the path of the ultraviolet laser source and the scanning camera.
All of the embodiments above employ an objective scan lens system made entirely of fused silica. A preferred lens system useful in the present invention has an objective end and an opposing image end, and is comprised of, from the objective end to the image end:
a. a first substantially plano-convex lens element, with its substantially piano side adjacent the objective end having a first concave surface facing the objective end and a second convex surface facing the image end and wherein the ratio of the radius of curvature of the first concave to the radius of curvature of the second convex surface is between 9.5:1 and 11.5:1;
b. a second substantially plano-concave lens element, with its substantially piano side adjacent the objective end wherein the second lens is positioned proximate to the first lens element having a first concave surface facing the objective end and a second concave surface facing the image end and wherein the ratio of the radius of curvature of the first surface to the radius of the curvature of the second surface is between about 9:1 and 11:1;
c. a third substantially plano-convex lens element, with its substantially plano side adjacent the image end wherein the third lens element is positioned proximate to the second lens element having first convex surface facing the objective end and a second convex surface facing the image end and wherein the ratio of the radius of curvature of the first surface to the radius of curvature of the first surface to the radius of curvature of the second surface is between about 1:3.5 and 1:4.3; and,
d. a fourth bi-concave lens element wherein the fourth lens element is positioned proximate to the third lens element having a first surface facing the objective end and a second surface facing the image end and wherein the ratio of the radius of curvature of the first surface to the radius of curvature of the second surface is between about 1:2.3 and 1:2.9, and wherein the first, second, third and fourth lens elements are sequentially aligned.
Other features and advantages of the invention will become more apparent upon a reading of the following detailed description together with the drawings in which like reference numerals refer to like parts.